With respect to the size (diameter) of golf balls, a diameter of not less than 1.680 inches (42.67 mm) is limited by a standard established by USGA (United States Golf Association) rule. Generally, when the diameter of the golf ball is large, air resistance of the golf ball on a flight is large, which reduces the flight distance. Therefore, most of golf balls which have been commercially available are designed to reduce its diameter to the lower limit of the standard as long as possible. The golf ball, of which the diameter is approached to the lower limit of the standard, is called as a standardized ball.
Although it is disadvantage in flight distance that the larger the ball size, the larger the air resistance when the ball is traveling, the larger sized golf ball has an advantage in that one feels one's target larger and easier to hit the ball with a sweet spot of a golf club, even in mental feeling. It is also known that when the larger sized golf ball is hit, the golf ball has large moment of inertia and is superior in straightness and retention of spin. The golf balls having a larger diameter (hereinafter called as "over-size golf ball") in order to obtain the advantage are described in, for example, Japanese Patent Kokai Publication Nos. 371170/1992, 114123/1994, 312032/1994, 80360/1996, 211301/1998 and the like.
In Japanese Patent Kokai Publication No. 211301/1998, an over-size golf ball having a suitable hardness and thickness of cover is described. In Japanese Patent Kokai Publication Nos. 371170/1992, 114123/1994, 312032/1994 and 80360/1996, over-size golf balls having an adjusted ratio of dimple area to surface area of the golf ball are described. In the golf balls described above, it is attempted to improve flight distance by adjusting the hardness and thickness of cover and the ratio of dimple area to surface area of the golf ball to a proper range, in combination with enlarging the diameter. However, when considering the reduction of the flight distance by increasing air resistance because of the enlargement of the diameter, the technical effect of extending the flight distance is not sufficiently obtained, and it is required to improve more. In other words, when an over-size golf ball is produced merely by enlarging a size of a standardized golf ball, an air resistance to the traveling golf ball is too much and severely reduces its flight distance. Accordingly, it is not sufficient to merely improve a standardized golf ball for obtaining an over-size golf ball.
On the other hand, the present inventors have developed a hollow solid golf ball having a hollow portion in a core (Japanese Patent Kokai Publication No. 127815/1998). Solid golf balls having no hollow portion have hard and poor shot feel and are not generally approved of or employed by professional golfers and the like. In order to improve the shot feel, it is effective to soften the solid core, but it is a drawback that the rebound characteristics are degraded, and reduces the flight distance. In order to improve the drawback, the present inventors have proposed the hollow solid golf ball having a hollow portion at the center of a core. Since the hollow solid golf ball comprises the hollow portion therein,
(1) an impact force at the time of hitting is small, and shot feel is good, and
(2) a moment of inertia is large, thus spin amount immediately after hitting is small, and retention of spin is large after the golf ball passes the highest point of the flight curve of the golf ball, which improves the flight distance. That is, the hollow solid golf ball is superior in both shot feel and flight distance.